I think our first fifthwheel used a 347 key. Or close to that number. But the last 3 all used the 751. As Carol said, nearly all use the 751 these days. A few high dollar rigs use a different key.
In any case, the locker door design does not lend itself to keeping those door secure. I won't spend money on different locks until I find something that will not let the door pop open by a brief prying with a simple screwdriver. I have on more than one occasion opened ours that way. The only damage was a slightly bent lever on the lock that was easily returned to original position with a little finger pressure. If you don't believe how easy it is, try it.